Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention, and/or to assess its effectiveness. IVUS imaging uses ultrasound echoes to create an image of the vessel of interest. Ultrasound waves pass easily through most tissues and blood, but they are partially reflected from discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the IVUS catheter by way of a patient interface module (PIM), processes the received ultrasound echoes to produce a cross-sectional image of the vessel where the catheter is placed.
There are two types of IVUS catheters in common use today: rotational and solid-state, with each having advantages and disadvantages. The rotational IVUS catheter typically incorporates a rotating transducer element to direct an ultrasound beam in different directions in order to collect the echo data required to form an image, and a rotary transformer or other rotating electromechanical interface is needed to provide an electrical connection in order to maintain communication between the IVUS imaging system and the rotating transducer element. In contrast, the solid-state IVUS catheter uses electronic circuitry to steer an ultrasound beam in different directions, and hence it does not require a rotary transformer or other rotating electromechanical interface.
In a typical rotational IVUS catheter, a single ultrasound transducer element is located near the tip of a flexible driveshaft that spins inside a plastic sheath inserted into the vessel of interest. The fluid-filled sheath protects the vessel tissue from the spinning driveshaft while permitting ultrasound signals to freely propagate from the transducer into the tissue and back. In a side-looking IVUS device, the transducer element is oriented such that the ultrasound beam propagates generally perpendicular to the axis of the catheter. In a forward-looking IVUS device, the transducer element is oriented such that the ultrasound beam propagates distally from the tip at an oblique angle relative to the axis of the catheter. In either case, as the driveshaft rotates (typically at up to 30 revolutions per second), the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. Immediately following the ultrasound transmit burst, the transducer listens for the returning echoes reflected from various tissue structures, and the IVUS imaging system assembles a two-dimensional display of the vessel cross-section from a sequence of several hundred of these pulse/acquisition cycles occurring during a single revolution of the transducer/driveshaft.
Typically, the rotational IVUS catheter includes a driveshaft disposed within the catheter body, with the ultrasound transducer attached near the distal tip of the driveshaft. A single element piezoelectric transducer requires only two electrical leads, with this single pair of leads delivering the intermittent electrical transmit pulses to the transducer, and returning the received echo signals from the transducer to the receiver amplifier during the intervals between transmit pulses. The IVUS catheter is coupled to an interface module, which typically controls the rotation of the drive shaft within the catheter body and contains the transmitter and receiver circuitry. Since the catheter driveshaft and transducer are spinning (in order to scan a cross-section of the artery) and the transmitter/receiver circuitry is stationary within the interface module, an electromechanical interface must be provided where the electrical signal traverses the rotating mechanical junction. As commonly implemented in rotational IVUS imaging systems and further described in one the present applicant's previous patent application, U.S. Patent Publication Application No. 2010/0234736 A1, filed on Mar. 11, 2009, which is hereby incorporated by reference in its entirety, this can be accomplished via a rotary transformer that comprises two halves, separated by a narrow air gap that permits electrical coupling between the primary and secondary windings of the transformer while allowing relative motion (rotation) between the two halves. The spinning element (transducer, electrical leads, and driveshaft) is attached to the spinning portion of the rotary transformer, while the stationary transmitter and receiver circuitry contained in the interface module are attached to the stationary portion of the rotary transformer. This allows the transmit pulses to be delivered to the transducer and the received echo signals from the transducer to be return across the rotating interface to the imaging system by way of a patient interface module (PIM). The IVUS imaging system then processes the echo signals and assembles the data into a cross-sectional image of the vessel.
Current rotational IVUS PIMs typically incorporate a rotary transformer constructed from hand-wound wire coils. With these hand-wound coils, it is difficult to precisely control the geometry of the coils and the alignment of coils between the two halves of the rotary transformer. These difficulties translate into transformers that are expensive and complex to manufacture, suffer from poor coupling efficiency and/or narrow bandwidth, and exhibit significant variability among devices. This variability among transformers leads to inconsistent performance for the associated IVUS imaging systems, and the poor coupling efficiency and/or narrow transformer bandwidth degrades the image quality that should theoretically be available from the IVUS imaging system. Further, due to the complexity and expense of manufacturing traditional transformers, they do not lend themselves to disposable or one-time use applications.
While existing rotary transformers used with conventional rotational IVUS catheters deliver useful diagnostic information, there is a need for an improved rotary transformer design with improved coupling efficiency and wider bandwidth to provide greater signal-to-noise ratio and increased depth of penetration for more refined insight into the vessel condition. Furthermore, there is a need for a consistent, low-cost method for manufacturing rotary transformers to support the particular needs of IVUS applications where the rotary transformer is included as a part of the sterile, disposable IVUS catheter assembly. Accordingly, there remains a need for improved electromechanical interfaces for use in rotational intravascular ultrasound systems.